Most electricity today is generated by burning fossil fuels and producing steam which is then used drive a steam turbine that, in turn, drives an electrical generator. Unfortunately, however, the world's supply of fossil fuels is large, but finite. Exhaustion of low-cost fossil fuels will have significant consequences for energy sources as well as for the manufacture of plastics and many other things.
More serious are concerns about the emissions that result from fossil fuel burning. Fossil fuels constitute a significant repository of carbon buried deep under the ground. Burning them results in the conversion of this carbon to carbon dioxide, which is then released into the atmosphere. This results in an increase in the Earth's levels of atmospheric carbon dioxide, which enhances the greenhouse effect and contributes to global warming. Depending upon the particular fossil fuel and the method of burning, other emissions may be produced as well. Ozone, SO2, NO2 and other gases are often released, as well as particulate matter. Sulfur and nitrogen oxides contribute to smog and acid rain. Fossil fuels, particularly coal, also contain dilute radioactive material, and burning them in very large quantities releases this material into the environment, leading to low but real levels of local and global radioactive contamination. Coal also contains traces of toxic heavy elements such as mercury, arsenic and others. Mercury vaporized in a power plant's boiler may stay suspended in the atmosphere and circulate around the world.
An alternative source of renewable energy, solar cells, also referred to as photovoltaic cells, use the photovoltaic effect of semiconductors to generate electricity directly from sunlight. Their use has been rather limited because of high manufacturing costs. Disadvantageously, the manufacturing process also consumes considerable fossil fuels, resulting in pollution. Additionally, refined silicon required for the semiconductors is in short supply, making solar cells relatively costly. Solar electricity currently tends to be more expensive than electricity generated by other sources. Furthermore, solar energy is not available at night, may be unavailable due to weather conditions, and may be compromised during winter months; therefore, a storage or complementary power system is required for most applications.
Moreover, solar energy is inefficient. Expensive solar cells made from single crystal silicon are currently limited to about 25% efficiency because they are most sensitive to infrared light, and radiation in this region of the electromagnetic spectrum is relatively low in energy. Polycrystalline solar cells are made by a casting process in which molten silicon is poured into a mould and allowed to cool, then sliced into wafers. This process results in cells that are significantly less expensive to produce than single crystal cells, but whose efficiency is limited to less than 20% due to internal resistance at the boundaries of the silicon crystals. Amorphous cells are made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH4). This type of solar cell can be applied as a thin film to low cost substrates such as glass or plastic. Thin film cells have a number of advantages, including easier deposition and assembly, the ability to be deposited on inexpensive substrates, the ease of mass production, and the high suitability to large applications. Since amorphous silicon cells have no crystal structure at all, their efficiencies are presently only about 10% due to significant internal energy losses.
Another attractive alternative source of renewable energy, wind power, produces electricity from the flow of air over the surface of the earth. Wind rotates a rotor mechanically to an electric generator to produce electricity. Unlike solar cells, properly located wind turbines can generate the energy used in its construction within just months of operation. Greenhouse gas emissions and air pollution produced by construction of a wind turbine are small and declining. There are no emissions or pollution produced by operation of a wind turbine. Modern wind turbines are almost silent and rotate so slowly (in terms of revolutions per minute) that they are rarely a serious hazard to birds. Aesthetic, landscape and heritage issues may be a significant issue for certain wind farms. However, when appropriate planning procedures are followed, these risks are minimal and should be weighed against the need to address the threats posed by climate change and the opinions of the broader community.
Unfortunately, conventional wind turbines suffer several serious shortcomings. For example, they rely exclusively on ambient wind speed. Nothing is done to accelerate the wind and thereby attempt to improve efficiency of the turbine. Known prior art wind energy systems do not include a shroud.
Another shortcoming is serviceability. Conventional rotors, blades and adjacent components are unreachable from the tower for maintenance. Mechanisms configured to retract the rotor towards a tower, or a yaw assembly configured to facilitate rotational movement of the shroud and rotor to face oncoming wind.
Yet another shortcoming is the stationary structure supporting the nacelle. While many wind turbines include yaw mechanisms to orient the nacelle into the wind, no prior art wind turbine rotates the structure supporting the nacelle. Thus the structure is stationary and typically designed with a circular or similar cross section, which exhibits similar aerodynamic properties from every angle. Such designs exhibit marginal aerodynamic performance, making the structure more prone to drag and susceptible to failure than a streamlined structure.
Still another shortcoming of conventional wind turbines is the required blade size to drive a particular generator. As conventional wind turbines do nothing to augment wind speed, power requirements are met by sizing the rotor. A large generator, of course, requires substantial power provided by a large rotor to turn. This approach ignores the relationship of wind speed to power, whereby an increased wind speed augments power output. Disadvantageously, a larger rotor increases manufacturing and construction costs, stresses on the support structure, wear and tear on bearings, and maintenance costs.
The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.